Valve spring

Abstract

A valve spring includes a nitrided layer, and a core portion that is further inward than the nitrided layer. A chemical composition of the core portion consists of, in mass %, C: 0.53 to 0.59%, Si: 2.51 to 2.90%, Mn: 0.70 to 0.85%, P: 0.020% or less, S: 0.020% or less, Cr: 1.40 to 1.70%, Mo: 0.17 to 0.53%, V: 0.23 to 0.33%, Ca: 0.0001 to 0.0050%, Cu: 0.050% or less, Ni: 0.050% or less, Al: 0.0050% or less, Ti: 0.050% or less, and N: 0.0070% or less, with the balance being Fe and impurities. In the core portion, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 500 to 8000 per μm 2 , and in the core portion, a numerical proportion of Ca sulfides with respect to a total number of oxide-based inclusions and sulfide-based inclusions is 0.20% or less.

The invention claimed is: 
     
       1. A valve spring, comprising:
 a nitrided layer formed in an outer layer, and 
 a core portion that is further inward than the nitrided layer; 
 wherein: 
 a chemical composition of the core portion consists of, in mass %, 
 C: 0.53 to 0.59%, 
 Si: 2.51 to 2.90%, 
 Mn: 0.70 to 0.85%, 
 P: 0.020% or less, 
 S: 0.020% or less, 
 Cr: 1.40 to 1.70%, 
 Mo: 0.17 to 0.53%, 
 V: 0.23 to 0.33%, 
 Ca: 0.0001 to 0.0050%, 
 Cu: 0.050% or less, 
 Ni: 0.050% or less, 
 Al: 0.0050% or less, 
 Ti: 0.050% or less, 
 N: 0.0070% or less, and 
 Nb: 0 to 0.020%, 
 with the balance being Fe and impurities; 
 in the core portion, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 500 to 8000 pieces/μm 2 ; and 
 when, among inclusions in the core portion, 
 inclusions in which, in mass %, an O content is 10.0% or more are defined as oxide-based inclusions, 
 inclusions in which, in mass %, an S content is 10.0% or more and the O content is less than 10.0% are defined as sulfide-based inclusions, and 
 among the sulfide-based inclusions, inclusions in which, in mass %, a Ca content is 10.0% or more, the S content is 10.0% or more, and the O content is less than 10.0% are defined as Ca sulfides, 
 in the core portion, a numerical proportion of the Ca sulfides with respect to a total number of the oxide-based inclusions and the sulfide-based inclusions is 0.20% or less. 
 
     
     
       2. The valve spring according to  claim 1 , wherein:
 in the chemical composition of the core portion: 
 an Nb content is 0.005 to 0.020%.